The present invention relates generally to apparatus for sensing and measuring pressures and more particularly to apparatus for sensing and measuring the differential between two fluid pressures.
Differential pressure sensing devices have many applications, but they are especially useful in the control of industrial processes. For example, they can be used to determine the gauge pressure of a fluid by referencing the pressure of the fluid against atmospheric pressure, to determine the absolute pressure of the fluid by referencing the fluid's pressure to a vacuum or absolute zero pressure, or to obtain the liquid head of the contained liquid by sensing the pressures above and below the surface of the liquid. Another application of such devices is to inferentially determine the rate, volume and/or mass of a fluid flowing in a line. Typically, this is accomplished by measuring the differential pressure of the fluid on opposite sides of a metering restriction, such as an orifice plate in the line, and then electronically processing the differential pressure measurement to determine flow rate, volume and/or mass of the fluid.
Numerous types of devices have been developed and commercialized for these purposes. One such type of device utilizes a differential pressure cell having two opposing diaphragms which interact with the fluids and respond to the fluid pressures being measured, and includes means for measuring the difference in pressure applied to the two diaphragms. One particular device of this type incorporates a third diaphragm which is positioned between the other two and which, in response to a differential pressure, is displaced towards the lower pressure. This diaphragm acts with other sensors within the cell to create a difference in capacitance which can be related to the differential pressure.
The use of a vibratory piezoelectric element, such as a quartz crystal to measure different parameters such as pressure is also known. An example of this technology is disclosed in U.S. Pat. No. 4,526,480, issued to Ward. The use of piezoelectric elements provides several advantages, but particularly increased sensitivity, resolution and stability as compared to other prior art devices. Accordingly, it would be a significant advancement in the art to provide a differential pressure sensing apparatus utilizing a vibratory, piezoelectric element to provide high sensitivity, resolution and stability at a relatively low cost. It would be a further significant advancement if such an apparatus was not responsive to the common mode value of the two pressures differenced and was not sensitive to changes in temperature which could give erroneous readings. Such an apparatus is disclosed and claimed herein.